The binding of Ku antigen to homeodomain proteins promotes their phosphorylation by DNA-dependent protein kinase

The Ku antigen (70- and 80-kDa subunits) is a regulatory subunit of DNA-dep endent protein kinase (DNA-PK) that promotes the recruitment of the catalyt ic subunit of DNA-PK (DNA-PKcs) to DNA ends and to specific DNA sequences f rom which the kinase is activated. Ku and DNA-PKcs plays essential roles in double-stranded DNA break repair and V(D)J recombination and have been imp licated in the regulation of specific gene transcription. In a yeast two-hy brid screen of a Jurkat T cell cDNA library, we have identified a specific interaction between the 70-kDa subunit of Ku heterodimer and the homeodomai n of HOXC4, a homeodomain protein expressed in the hematopoietic system. Un expectedly, a similar interaction with Ku was observed for several addition al homeodomain proteins including octamer transcription factors 1 and 2 and D1x2, suggesting that specific binding to Ku may be a property shared by m any homeodomain proteins. Ku-homeodomain binding was mediated through the e xtreme C terminus of Ku70 and was abrogated by amino acid substitutions at Lys(595)/lys(596). Ku binding allowed the recruitment of the homeodomain to DNA ends and dramatically enhanced the phosphorylation of homeodomain-cont aining proteins by DNA-PK. These results suggest that Ku functions as a sub strate docking protein for signaling by DNA-PK to homeodomain proteins from DNA ends.